Add an extra null check to Channel.GetUser when permission cache is disabled

src/Discord.Net/Models/Channel.cs

@@ -563,13 +563,25 @@ namespace Discord
         public User GetUser(ulong id)
         {
             if (!Client.Config.UsePermissionsCache)
+            {
+                if (Server != null)
                 {
                     var user = Server.GetUser(id);
+                    if (user != null)
+                    {
                         ChannelPermissions perms = new ChannelPermissions();
                         UpdatePermissions(user, perms);
                         if (perms.ReadMessages)
                             return user;
+                    }
+                }
                 else
+                {
+                    if (id == Recipient.Id)
+                        return Recipient;
+                    else if (id == Client.PrivateUser.Id)
+                        return Client.PrivateUser;
+                }
                 return null;
             }